Under a variety of physiological and pathological circumstances, cells die by apoptosis or programmed cell death, an internally programmed process of orderly cell suicide which is under genetic and biochemical control. We are investigating the molecular mechanisms of apoptosis in several systems. In one study, we have characterized the cell death of proliferating and differentiating stem cells of 16-18-day fetal forebrain upon exposure to low doses of ionizing radiation in utero. We found that gamma- radiation (2-4 Gy) elicits within 5 hours nuclear pyknosis and fragmentation in 80% of cortical neuroepithelial cells and 50% of cortical-plate neurons. Fetal brain genomic DNA is fragmented within 3 hours into an oligonucleosomal ladder pattern which is a hallmark of apoptosis. These morphological changes and DNA fragmentation require ongoing RNA and protein synthesis, as determined by the use of specific inhibitors. These biochemical characteristics confirm previous inferences from microscopy that radiation kills most fetal brain cells by the mechanism of apoptosis rather than necrosis. Irradiation (4 Gy) caused the abundances of most mRNA species, including those of housekeeping genes, to decline by 50-70%. However, irradiation dramatically increased posttranscriptionally the level of the p53 transcription factor, and this in turn dramatically induced the mRNA for the p53-inducible Waf-1/Cip-1 cell-cycle arrestor. Radiation dramatically increased the levels of mRNAs for the c-Fos and JunB "immediate-early" gene products, without increasing the already high levels of mRNA for c-Jun and c-Myc. Irradiation increased by 3-4 fold Bax, a positive regulator protein of apoptosis, and its mRNA but decreased the mRNAs for Bcl-2 and Bcl-xL, two negative regulators of apoptosis. These results indicate that radiation-elicited apoptosis of fetal brain cells is associated with activation of the p53 system, probable increases in transcription factor AP-1 Fos/JunB heterodimers, and an increased ratio of Bax to [Bcl-2 + Bcl-xL] proteins. These inductions may be causally related to the triggering of apoptosis. Dr. Sabol has begun a sabbatical year in the laboratory of Jonathan Ashwell, NCI, NIH, to study novel molecular mechanisms of apoptosis in the 2B4 mouse T-cell hybridoma and Jurkat human lymphoid cell lines, which die by apoptosis in response to T-cell receptor activation, Fas ligation, or glucocorticoids. He is studying the cytoplasmic factor(s) generated by apoptotic signals which induce specific apoptotic changes and endonuclease activation in the nucleus.